Crucible molds

ABSTRACT

Methods for forming a unitized crucible assembly for holding a melt of silicon for forming a silicon ingot are disclosed. In some embodiments, the methods involve a porous crucible mold having a channel network with a bottom channel, an outer sidewall channel that extends from the bottom channel, and a central weir channel that extends from the bottom channel. A slip slurry may be added to the channel network and the liquid carrier of the slip slurry may be drawn into the mold. The resulting green body may be sintered to form the crucible assembly.

FIELD OF THE DISCLOSURE

The field of the disclosure relates to methods for forming a unitizedcrucible assembly for holding a melt of silicon for forming a siliconingot and, in particular, methods for forming a unitized crucibleassembly having central and inner weirs for use in continuousCzochralski silicon ingot growth. The field of the disclosure alsorelates to crucible molds and unitized crucibles.

BACKGROUND

Single crystal silicon ingots may be prepared by the so-calledCzochralski method in which a single crystal silicon seed is contactedwith a silicon melt held within a crucible. The single crystal siliconseed is withdrawn from the melt to pull the single crystal silicon ingotfrom the melt. The ingot may be prepared in a batch system in which acharge of polycrystalline silicon is initially melted within thecrucible and the silicon ingot is withdrawn from the melt until themelted silicon within the crucible is depleted. Alternatively, the ingotmay be withdrawn in a continuous Czochralski method in which polysiliconis intermittently or continuously added to the melt to replenish thesilicon melt during ingot growth.

In a continuous Czochralski method, the crucible may be divided intoseparate melt zones. For example, the crucible assembly may include anouter melt zone in which polycrystalline silicon is added and melted toreplenish the silicon melt as the silicon ingot grows. The silicon meltflows from the outer melt zone to a stabilization zone within the outermelt zone in which the melt thermally stabilizes. The silicon melt thenflows from the stabilization zone to a growth zone from which thesilicon ingot is pulled.

Conventionally, crucible assemblies for growing silicon ingots by acontinuous Czochralski method may include one or more nested cruciblessuch as the crucible assembly shown in U.S. Pat. No. 10,450,670.Alternatively or in addition, the crucible assembly may include one ormore weirs that are connected to and extend upward from the bottom ofthe crucible as disclosed in U.S. Pat. No. 8,262,797. Both types ofcrucible arrangements involve use of alignment tools when placed intothe crystal puller hot zone to provide uniform gas flow over thecrucible assembly. Both types of constructions require a connection beformed at the bottom of the crucible assembly (i.e., boding of weirs orcrucibles to the bottom of the assembly). These connections may degradewhich causes the crucible assembly to deform and lose one or moretolerances.

A need exists for crucible assemblies that resist deformation, that maybe placed in the hot zone without or with less alignment tools, andwhich may be more simply fabricated, and for methods for preparing suchcrucible assemblies.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the disclosure, which aredescribed and/or claimed below. This discussion is believed to behelpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

SUMMARY

An aspect of the present disclosure is directed to a method for forminga unitized crucible assembly for holding a melt of silicon for forming asilicon ingot by the Czochralski method. A crucible mold is provided.The mold has a channel network that includes a bottom channel and anouter sidewall channel that extends from the bottom channel. The channelnetwork also includes a central weir channel that extends from thebottom channel and an inner weir channel that extends from the bottomchannel. The central weir channel is disposed between the outer sidewallchannel and the inner weir channel. A slip slurry is introduced into thechannel network to fill the bottom channel, outer sidewall channel,central weir channel, and inner weir channel with the slip slurry. Theslip slurry includes silica and a liquid carrier. The liquid carrier isat least partially removed from the channel network to form a greenbody. The green body is removed from the crucible mold. The green bodyis sintered to dry and densify the green body to form the unitizedcrucible assembly.

Another aspect of the present disclosure is directed to a method forforming a unitized crucible assembly for holding a melt of silicon forforming a silicon ingot by the Czochralski method. A crucible mold isprovided. The mold includes a porous body and a channel network disposedwithin the porous body. The channel network includes a bottom channel,an outer sidewall channel that extends from the bottom channel, and aninner weir channel that extends from the bottom channel. The inner weirchannel is disposed interior to the outer sidewall channel. A slipslurry is introduced into the channel network to fill the bottomchannel, outer sidewall channel and inner weir channel with the slipslurry. The slip slurry includes silica and a liquid carrier. The liquidcarrier is at least partially drawn into the mold by capillary action toform a green body. The green body is separated from the crucible mold.The green body is sintered to dry and densify the green body to form theunitized crucible assembly.

Yet another aspect of the present disclosure is directed to a cruciblemold. The crucible mold includes a lower portion and an upper portiondisposed above the lower portion. The upper portion forms a central weirchannel and an inner weir channel. The upper portion and lower portiontogether form a bottom channel and a sidewall channel fluidly connectedto the bottom channel. The central weir channel and inner weir channelextend from the bottom channel.

Yet a further aspect of the present disclosure is directed to a crucibleassembly. The crucible assembly includes a bottom, an outer sidewallthat extends upward from the bottom, a central weir that extends upwardfrom the bottom and an inner weir that extends upward from the bottom.The central weir is disposed between the outer sidewall and the innerweir. The bottom, outer sidewall, central weir and inner weir areunitized and are free of seams at (1) a joint between the outer sidewalland the bottom, (2) a joint between the central weir and the bottom, (3)a joint between the inner weir and the bottom.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an embodiment of a unitized crucibleassembly;

FIG. 2 is a perspective view of an embodiment of a crucible mold havinga green body formed therein:

FIG. 3 is a perspective cross-section view of the crucible mold andgreen body taken along line 3-3 in FIG. 2;

FIG. 4 is a top view of the crucible mold and green body;

FIG. 5 is a cross-section view of the crucible mold without the greenbody taken along line 5-5 in FIG. 4;

FIG. 6 is another cross-section view of the crucible mold without thegreen body taken along line 6-6 in FIG. 4;

FIG. 7 is a cross-section view of the crucible mold having a slip slurrypoured therein;

FIG. 8 is another cross-section view of the crucible mold having a slipslurry poured therein; and

FIG. 9 is a detailed cross-section view of the crucible mold showing acentral weir peg and an inner weir peg for forming openings in a centralweir and an inner weir of the resulting crucible.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

Provisions of the present disclosure relate to methods for forming aunitized crucible (e.g., one-piece construction) for holding a melt ofsilicon for forming a silicon ingot by the Czochralski method. Anexample unitized crucible assembly 5 that may be produced by embodimentsof the present disclosure is shown in FIG. 1. The unitized crucibleassembly 5 includes a bottom 17 and an outer sidewall 10 that extendsupward from the bottom 17. The crucible assembly 5 includes a centralweir 24 and an inner weir 31 that both extend upward from the bottom 17.The central weir 24 is disposed between the outer sidewall 10 and theinner weir 31.

In accordance with embodiments of the present disclosure, a cruciblemold 8 (FIG. 2) having a channel network 20 (FIGS. 5-6) formed thereinis provided. The mold 8 includes an upper portion 12 and a lower portion18 which, when assembled together, form the channel network 20. Asdescribed further below, a slip is poured into the channel network 20and a “green body” 40 (FIG. 2) in the shape of the crucible is formed.The green body 40 is removed from the mold 8 and is sintered to form theunitized crucible assembly 5 (FIG. 1).

Referring now to FIGS. 2-4 in which the mold 8 is transparent forillustration and the channel network 20 is shown with a green body 40formed therein, the channel network 20 includes a bottom channel 30 andan outer sidewall channel 32. The channel network 20 also includes acentral weir channel 34 and an inner weir channel 36 that are eachdisposed inward of the outer sidewall channel 32. The central weirchannel 34 is disposed between inner weir channel 36 and the outersidewall channel 32. As shown in FIG. 3, the outer sidewall channel 32,central weir channel 34 and the inner weir channel 36 are fluidlyconnected to the bottom channel 30 (i.e., each of the channels of thechannel network are fluidly connected which allow the resulting crucibleto be unitized).

The upper portion 12 of the mold 8 includes a main body 49 (FIGS. 5-6)and a flange 54 that extends radially outward from the main body 49. Themain body 49 has a lower surface 53 (i.e., the surface below the flange54 that forms a surface of the bottom channel 30 and the outer sidewallchannel 32).

The lower portion 18 of the mold 8 also includes a main body 56. Themain body 56 has an upper surface 59 (i.e., the surfaces that form asurface of the bottom channel 30 and the outer sidewall channel 32) andforms a recess 61 inward of the upper surface 59. In the illustratedembodiment, at least a portion of the lower surface 53 of the main body49 of the upper portion 12 of the mold 8 and at least a portion of theupper surface 59 of the lower portion 18 of the mold 8 are rounded toform a rounded crucible bottom 17 (FIG. 1). To assemble the mold 8, themain body 49 of the upper portion 12 is lowered into the recess 61 ofthe lower portion 18 until the flange 54 rests on the lower portion 18.In this position, the upper and lower portions 12, 18 of the mold 8together form the bottom channel 30 and the outer sidewall channel 32.

In the illustrated embodiment, the bottom channel 30 is rounded (and theresulting crucible bottom 17 (FIG. 1) is rounded). In other embodiments,the bottom channel 30 may be more or fully horizontal (i.e., flat) ormay be conical or inverted (e.g., having a pedestal in the middle). Asshown in FIGS. 5-6, a portion of the sidewall channel 32 is higher thanthe central weir channel 34 and a portion of the central weir channel 34is higher than the inner weir channel 36. In other embodiments, thechannels may have different relative heights (e.g., the same height ofsidewall channel 32, central weir channel 34 and inner weir channel 36).

First and second outer sidewall channel risers 58, 60 (FIG. 2) extendthrough the upper portion 12 and are in fluid communication with theouter sidewall channel 32. First and second central weir channel risers64, 66 also extend through the upper portion 12 and are in fluidcommunication with the central weir channel 34. First and second innerweir channel risers 70, 72 also extend through the upper portion 12 andare in fluid communication with the inner weir channel 36. The risers58, 60, 64, 66, 70, 72 extend through the upper portion 12 to allow thechannel network 20 to be filled with the slip slurry. For example, theslip slurry may be added to either the first and second sidewall riser58, 60 to fill the outer sidewall channel 32 and the bottom channel 30with slurry. As the channels 30, 32 fill with slurry, air escapesthrough the corresponding riser 58, 60 that is not being filled withslip slurry. Similarly, slip slurry may be added to either the first orsecond central weir channel risers 64, 66 and to the first or secondinner weir channel risers 70, 72 until the channel network 20 is filledwith slip slurry (FIGS. 7-8). In the illustrated embodiment, each riser58, 60, 64, 66, 70, 72 includes a cone portion 51 (FIG. 5) that opens tothe surface 57 of the upper portion 12 of the mold 8 to facilitatepouring the slip slurry into the riser. A duct 52 below the cone portion51 fluidly connects the cone portion 51 to the respective channel. Theslip slurry may be added until the ducts 52 and/or cone portion 51begins to fill to ensure that the channel network 20 is filled with slipslurry. For example, as the liquid carrier is drawn into the mold 8, thevolume of the slip slurry shrinks. Slip slurry disposed within the ducts52 and/or cone portion 51 of one or more of the risers 58, 60, 64, 66,70, 72 may be drawn into the channel network 20 as the slip slurryshrinks.

The channels may generally be filled in any order that allows thechannel network 20 to fill with slip slurry before parts of the slurrybegin to solidify as a green body to allow the resulting crucible to beunitized. The arrangement of risers 58, 60, 64, 66, 70, 72 is exemplaryand other arrangements may be used unless stated otherwise. The risers58, 60, 64, 66, 70, 72 may be openings and/or chambers formed in theupper portion 12 of the mold 8. In some embodiments, the risers 58, 60,64, 66, 70, 72 include openings/chambers and a liner that is disposed onthe surface of the openings/chambers.

In some embodiments, the slip slurry that is added to the channelnetwork 20 to fill the bottom channel 30, outer sidewall channel 32,central weir channel 34 and inner weir channel 36 includes silica and aliquid carrier such as water. The slip slurry may also include otherreagents such as suspending agents that keep the silica particles insuspension including any of the suspending agents known to those ofskill in the art. Example suspending agents include polymers or organicsthat absorb onto the particles (e.g., long-chain organic molecules orother agents that allow a surface charge to build-up on the silicaparticles to reduce particle to particle contact). The slip slurry mayalso include one or more binding agents that may optionally burn offduring sintering as described below. Optionally the slip slurry mayinclude one or more release agents to promote separation of the cruciblemold 8 from the resulting green body 40.

The crucible mold 8 may be made of materials that allow the liquidcarrier to be removed from the channel network 20 (e.g., such as bycapillary action) to form the green body 40. In some embodiments, thecrucible mold 8 is made of a plaster such as gypsum plaster (e.g.,CaSO₄.nH₂O which may also be referred to as plaster of Paris). In otherembodiments, the crucible mold 8 is made of porous silica. The cruciblemold 8 may be a generally porous body that draws the liquid carrier intothe mold 8 by capillary action. In other embodiments, the liquid carriermay be drawn out by vacuum.

Once the liquid carrier is drawn out of the slip slurry and into themold 8, a “green body” 40 (FIGS. 2-3) remains in the mold 8. Forexample, the green body 40 may have sufficient structure to maintain itsshape when separated from the mold. For example, the moisture content ofthe green body may be less than about 50%, less than about 45%, at leastabout 30 wt %, at least about 35 wt %, at least about 40 wt %, at leastabout 45 wt %, from about 30 wt % to about 50 wt % or from about 35 wt %to about 45 wt %.

The green body 40 may be further dried such as by exposing the greenbody 40 to a relatively low and/or controlled humidity ambient (e.g.,after the green body 40 has sufficient strength, the mold 8 is removedand the green body 40 is exposed to the relatively low and/or controlledhumidity ambient). The term “green body” or “green state” as used hereinshould not be considered in a limiting sense and generally refers to anintermediate state of the crucible after the liquid carrier has beenpartially drawn from the slip slurry and before sintering of thestructure.

To separate the mold 8 from the green body 40, the upper portion 12 ofthe mold 8 may be lifted from the green body 40 and from the lowerportion 18. The green body 40 may then be lifted from the lower portion18. The resulting green body 40 may have projections (not shown) thatextend up from the sidewalls and weirs that correspond to the riserlocations (e.g., the slip slurry was added to completely fill thechannel network 20 such that an additional amount filled in the risers).These projections may be ground or cut from the green body 40 or theresulting crucible assembly 5 (FIG. 1).

In some embodiments, the mold 8 may include pegs in one or more of thechannels to form openings within the resulting crucible to allow moltensilicon to move between the various sections of the crucible. Forexample and with reference to FIG. 9, the upper portion 12 includes acentral weir peg 48 that extends laterally across the central weirchannel 34. The slip slurry flows around the central weir peg 48 whichcreates an opening 38 (FIG. 1) through the central weir 24 of theresulting crucible assembly 5. Alternatively or in addition, the upperportion 12 includes an inner weir peg 55 that extends laterally acrossthe inner weir channel 36 to form an opening 41 (FIG. 1) in the innerweir 31. After the upper portion 12 of the mold 8 is removed from thegreen body 40 and the lower portion 18, the central weir peg 48 and theinner weir peg 55 may be removed (e.g., drilled or chiseled out). Thepositon of the pegs 48, 55 (and resulting crucible openings 38, 41) isexemplary and other positions (e.g., heights, relative circumferentialpositions between pegs, and the like). The mold 8 may include additionalor other arrangements of pegs to form openings in the central weir 24and/or inner weir 31. In some embodiments, the mold 8 does not includepegs. For example, the openings between weirs may be formed directly inthe green body and/or resulting crucible (e.g., by drilling orchiseling) and/or the crucible assembly 5 is configured such thatsilicon pours over the weirs.

The crucible mold 8 is exemplary and other embodiments may containdifferent arrangements of weirs and the like unless stated otherwise(e.g., containing only a single weir (i.e., only an inner weir) or morethan two weirs). In some embodiments, the crucible mold 8 may be re-usedin additional cycles for forming a crucible assembly 5 (e.g., used intwo, three, four, five or ten or more cycles). The mold 8 may be driedbetween cycles such as by placing the mold in a drying oven to evaporatethe liquid that was pulled therein during formation of the green body40.

Once the green body 40 is removed from the mold 8, the green body 40 maybe sintered (e.g., in a drying furnace) to dry and densify the greenbody 40 and to form the unitized crucible assembly 5 (FIG. 1). The greenbody 40 may be sintered at a temperature from about 1200° C. to about1800° C., from about 1300° C. to about 1700° C., or from about 1300° C.to about 1650° C. In some embodiments, the crucible assembly 5 has amoisture content of less than 20 wt %, less than 15 wt % or less than 10wt % after sintering.

The unitized crucible assembly 5 after sintering is shown in FIG. 1. Invarious embodiments of the present disclosure, the unitized crucibleassembly 5 does not include seams at the joint 13 formed between thebottom 17 and the central weir 24 and the joint 15 formed at the bottom17 and the inner weir 31 (i.e., without voids at these joints 13, 15).Alternatively or in addition, the unitized crucible assembly 5 does notinclude tacking at the joints 13, 15. Such tacking may be used inconventional crucible assemblies to connect the weirs 24, 31 at thecrucible bottom 17. In the illustrated embodiment, the unitized crucibleassembly 5 includes a single layer bottom 17 (i.e., the crucibleassembly 5 does not include stacked crucibles each having its ownbottom). The unitized crucible assembly 5 may be transparent.

In some embodiments, the slip slurry is selected such that the resultingcrucible assembly has a desired purity threshold. For example and inaccordance with some embodiments, the crucible assembly 5 includescalcium in a concentration less than about 1 ppmw, less than about 0.8ppmw or less than about 0.7 ppmw. Alternatively or in addition, thecrucible assembly 5 may include sodium at a concentration of less thanabout 0.5 ppmw, less than about 0.2 ppmw or less than about 0.1 ppmw.Alternatively or in addition, the crucible assembly 5 may includepotassium at a concentration of less than about 0.5 ppmw, less thanabout 0.2 ppmw or less than about 0.1 ppmw. Alternatively or inaddition, the crucible assembly 5 may include lithium at a concentrationof less than about 0.5 ppmw, less than about 0.4 ppmw or less than 0.3ppmw. Alternatively or in addition, the crucible assembly 5 may includeiron at a concentration of less than about 0.5 ppmw, less than about 0.3ppmw or less than about 0.15 ppmw.

In some embodiments, the mold 8 is selected (and/or processed) toachieve one or more of the purity amounts listed above. For example, themold may be made of porous silica to reduce calcium content of theresulting crucible (e.g., relative to gypsum plaster molds).

The unitized crucible assembly 5 shown and described herein is anexample assembly. The assembly 5 may have other dimensions (e.g., ashallower or deeper melt in one or more melt zones), purities, featuresand/or arrangements unless stated otherwise.

The methods of the present disclosure for forming a unitized cruciblemay be used to produce a single crystal silicon ingot. In such methods,a unitized crucible produced by an embodiment of the methods of thepresent disclosure is provided. In some embodiments, the unitizedcrucible assembly 5 includes a crucible melt zone 22 disposed betweenthe outer sidewall 10 and the central weir 24. The unitized crucibleassembly 5 also contains a stabilization zone 26 disposed between thecentral weir 24 and the inner weir 31. The unitized crucible assembly 5also contains a growth zone 28 disposed within the inner weir 31.

Polycrystalline silicon is added to the crucible melt zone 22 where thesilicon melts and replenishes the silicon melt. Silicon melt flowsthrough the central weir opening 38 and into the stabilization zone 26.The silicon melt then flows through the inner weir opening 41 to thegrowth zone 28 disposed within the inner weir 31. The silicon meltwithin the growth zone 28 is contacted with a single seed crystal andthe seed crystal is withdrawn from the silicon melt to form a singlecrystal silicon ingot. The various silicon melt zones (e.g., melt zone22, stabilization zone 26 and growth zone 28) allow the ingot to begrown in accordance with continuous Czochralski methods in whichpolycrystalline silicon is continuously or semi-continuously added tothe melt while an ingot is continuously pulled from the growth zone 28.

Compared to conventional crucible assemblies for holding a silicon melt,the crucible assemblies and methods for producing such crucibleassemblies of the present disclosure have several advantages. Byproducing a unitized (e.g., one-piece construction) crucible having acentral weir and an inner weir, the crucible assembly allows forcontinuous ingot growth (continuous Czochralski) while allowing for theadvantages of a unitized crucible. By unitizing the crucible bottom,sidewalls and weirs, the crucible assembly is better able to holdtolerances (e.g., height of sidewall and weirs, wall thickness, meltflow path, and the like). This allows the crucible assembly to moreeasily fit-up into the ingot puller system which may reduce or eliminateuse of alignment tools (i.e., tools to fit the various parts of anon-unitized crucible together within the ingot puller system). Theunitized crucible may undulate less which allows it to better fit intothe susceptor of the ingot puller apparatus. The unitized crucible alsoenables more wall thickness control for improved thermal response,allows for more flexibility for doped chemistries, and reduces oreliminates pre-bond cycles (i.e., heating cycles used to connect a weirto the crucible bottom) which consume ingot puller assembly run times.Unitized crucible assemblies also involve less processing and supplylogistics compared to crucible assemblies formed from multiple parts. Inembodiments in which the crucible mold includes pegs that extend acrossthe channels for forming weirs in the crucible, the resulting cruciblehas pre-formed openings which allow silicon melt to move to various meltzones within the resulting crucible during ingot growth.

In some embodiments, the slip and/or mold are selected such that theresulting crucible has a relatively low impurity content (e.g., lessthan about 1 ppmw calcium, less than about 0.5 ppmw sodium, less thanabout 0.5 ppmw potassium, less than 0.5 ppmw lithium, and/or less than0.5 ppmw iron) to reduce likelihood of loss of zero dislocation duringingot growth. In embodiments in which the mold is made of porous silica,the crucible assembly may be more pure due to the lower calcium contentof the porous silica relative to components cast from molds made ofgypsum plaster.

As used herein, the terms “about,” “substantially,” “essentially” and“approximately” when used in conjunction with ranges of dimensions,concentrations, temperatures or other physical or chemical properties orcharacteristics is meant to cover variations that may exist in the upperand/or lower limits of the ranges of the properties or characteristics,including, for example, variations resulting from rounding, measurementmethodology or other statistical variation.

When introducing elements of the present disclosure or the embodiment(s)thereof, the articles “a”, “an”, “the” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”“including,” “containing” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements. The use of terms indicating a particular orientation (e.g.,“top”, “bottom”, “side”, etc.) is for convenience of description anddoes not require any particular orientation of the item described.

As various changes could be made in the above constructions and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying drawing[s] shall be interpreted as illustrative and not ina limiting sense.

What is claimed is:
 1. A crucible mold comprising: a lower portion; anupper portion disposed above the lower portion, the upper portionforming: a central weir channel; and an inner weir channel; wherein theupper portion and lower portion together form:  a bottom channel; and  asidewall channel fluidly connected to the bottom channel, wherein thecentral weir channel and inner weir channel extend from the bottomchannel; and a central weir peg that extends laterally across thecentral weir channel to form an opening in a central weir of a crucibleformed from the mold.
 2. The crucible mold as set forth in claim 1wherein the upper portion comprises a flange, the flange resting on thelower portion.
 3. The crucible mold as set forth in claim 1 wherein thecrucible mold is porous.
 4. The crucible mold as set forth in claim 3wherein the crucible mold is made of porous silica.
 5. The crucible moldas set forth in claim 1 comprising an inner weir peg that extendslaterally across the inner weir channel to form an opening in an innerweir of a crucible formed from the mold.
 6. The crucible mold as setforth in claim 1 wherein the bottom channel is rounded.
 7. The cruciblemold as set forth in claim 1 wherein the upper portion has a main bodyhaving lower surface and the lower portion has a main body having anupper surface, the lower and upper surfaces forming the bottom channeland the sidewall channel.
 8. The crucible mold as set forth in claim 7wherein the upper portion comprises a flange that extends from the mainbody of the upper portion, the flange resting on the main body of thelower portion.
 9. The crucible mold as set forth in claim 7 wherein atleast a portion of the lower surface is rounded and at least portion ofthe upper surface is rounded.